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Date: Tue, 08 May 2012 17:26:55 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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To: "Michael S. Tsirkin" <mst@redhat.com>
CC: x86@kernel.org, kvm@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Avi Kivity <avi@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>, gleb@redhat.com,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC dontapply 5/5] kvm_para: guest side for eoi avoidance
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Il 23/04/2012 16:04, Michael S. Tsirkin ha scritto:
> +/* Our own copy of __test_and_clear_bit to make sure
> + * it is done with a single instruction */

Is this for microoptimization or correctness?  If the latter, it does
not ensure anything without a "lock" prefix.

Paolo

> +static inline int kvm_test_and_clear_bit(int nr, volatile u16* addr)
> +{
> +	int oldbit;
> +
> +	asm volatile("btr %2,%1\n\t"
> +		     "sbb %0,%0"
> +		     : "=r" (oldbit), BITOP_ADDR_CONSTRAINT(*addr) : "Ir" (nr));
> +	return oldbit;